This application claims the priority of Korean Patent Application Nos. 2002-63539 and 2003-33842, filed on Oct. 17, 2002 and May 27, 2003, respectively, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a semiconductor optoelectronic device, and more particularly, to a semiconductor optoelectronic device having a high optical efficiency.
2. Description of the Related Art
As shown in FIG. 1, a semiconductor laser device, which is one kind of nitride-based semiconductor optoelectronic device, has a sapphire substrate 1 as its base and includes an active layer 5 emitting or receiving light, n-type layers 2, 3, and 4 deposited on the sapphire substrate 1, and p-type layers 6, 7, and 8 deposited on the n-type layers 2, 3, and 4.
In particular, the p-type layer 7 and the n-type layer respectively provided on and under the active layer 5 are a p-type waveguide layer and an n-type waveguide layer, respectively. In order to confine electrons, holes, and light, the p-type cladding layer 8 and the n-type cladding layer 3 are provided on the p-type waveguide layer 7 and under the n-type waveguide layer 4, respectively. The electron blocking layer 6 through which only holes can pass is provided between the p-type waveguide layer 7 and the active layer 5. The n-type GaN contact layer 2 is provided between the sapphire substrate 1 and the n-type cladding layer 3, and a p-type GaN layer is provided on the p-type cladding layer 8.
Such a nitride-based emitting device uses a sapphire substrate. In general, in a nitride-based laser diode, a waveguide layer for guiding light emitted from an active layer is formed of GaN, and a cladding layer confining electrons and light therein is formed of AlxGa1−xN.
AlxGa1−xN has a greater energy gap than GaN. In addition, a difference in refractory indexes between AlxGa1−xN and InGaN, which forms an active layer, is greater than a difference in refractory indexes between GaN and InGaN.
AlxGa1−xN has smaller lattice constant and thermal expansion coefficient than GaN used as a material of an n-type contact layer, which may cause cracks in a nitride-based laser diode. Accordingly, there have been limits put on the composition ratio and thickness of aluminum in a cladding layer formed of AlxGa1−xN. According to various research results, those limits are as follows. A supper lattice formed of AlGaN/GaN has an aluminum composition ratio of about 14% and an aluminum thickness of 1 μm. Bulk AlGaN layer has an aluminum composition ratio of about 8% and an aluminum thickness of 1 μm. Due to those limits put on the composition ratio and thickness of aluminum in a cladding layer formed of AlxGa1−xN, mode leakage toward a substrate, i.e., an optical confinement factor, decreases.
Mode leakage reduces an optical confinement factor, and thus an optical gain decreases. Therefore, the leakage mode increases the amount of current necessary to start oscillating a laser device and has a bad influence on a far-field pattern of the laser device. A decrease in the optical confinement factor also increases the amount of current necessary to start oscillating the laser device, decreases an internal quantum efficiency, and also has a bad influence on the far-field pattern of the laser device.